This invention relates to improved ultraviolet light curable silicone release coating compositions. More particularly, it relates to novel additive blends of alkylphenols and alkyl vinyl ethers, which, when added to epoxysilicone polymers, provide for superior miscibility of polar onium salt photocatalysts in the epoxysilicone medium, and provide for superior release of the resulting photocured silicone coating from acrylic adhesives laminated thereto.
Silicone coating compositions are used for many applications including release coatings, or adhesive coatings, designed to permit release of aggressive adhesives without denigration of the adhesive properties due to contact with the silicone surface. Other silicone coatings well known to those versed in the art include protective coatings and conformal coatings. Such coatings have often been applied to various substrates as dispersions in solvents such as low boiling aromatic and aliphatic organic compounds or as emulsions dispersed in water. In such cases it is necessary to apply considerable heat to both remove the dispersing agent and to accelerate crosslinking (curing) of the silicone that has been applied to the substrate. Application of silicone without solvent or water dispersing agents is a desirable improvement of the art in that removal of volatile organic solvent requires environmentally compliant technology that adds cost to the coating process without otherwise benefiting the coating process. Similarly, elimination of a thermal cure step in favor of a low temperature radiation crosslinking process permits use of film, plastic or other thin gauge substrate material for the silicone release coating which would otherwise be ruled out because such substrates cannot accommodate the heat required for conventional silicone release coating crosslinking.
Silicone compositions have long been in use for rendering surfaces non-adherent to materials which would otherwise adhere thereto. Epoxy-functional polydimethylsiloxanes (PDMS), when combined with compatible iodonium cationic type photocatalysts, are well known to those skilled in the art to be useful UV curable silicone release coatings. Such combinations of organofunctional silicone polymers and lipophilic iodonium photocatalysts have been taught in U.S. Pat. No. 4,279,717 and many other subsequent patents. Such coatings based on cationic photocurable epoxy-silicones have been shown to be readily coatable to thin, defect-free continuous films when applied to smooth paper or plastic film liners using 3 roll offset gravure or multiroll liquid film splitting techniques, and these coatings are quickly crosslinked by exposure to focused ultraviolet light to create low surface energy adhesive surfaces capable of release of organic pressure sensitive adhesives (PSA). A drawback to the cationic epoxysilicone based photocurable release coating systems has been their inability to provide long term stable release from commonly used aggressive acrylic PSA""s, particularly if the adhesive is laminated to the silicone coating in-line, that is, as part of the converting process. While the precise cause of this performance deficiency is not established, it is thought that a significant post-cure of the epoxysilicone coating takes place which, when occurring in the laminate with the acrylic PSA, leads to chemical interactions of the silicone surface with the acrylic PSA which in turn leads to unstable release and even a lock-up of the silicone surface with the PSA. It is therefore clear that providing a means by which a photocured epoxysilicone release coating can consistently give low (premium) release of aggressive acrylic adhesives as a function of laminate aging is highly desirable.
The absence of solvent from a coating composition reduces the energy needed for crosslinking as well as the need for pollution abatement. But the absence of solvent leads to problems insofar as silicone release coatings are non-polar viscous materials that are immiscible with many polar additives including ""onium type cationic photocatalysts. Another problem associated with solvent-free silicone coatings is that the high viscosity associated with solvent free compositions is unsuited for many cost-effective coating techniques including direct gravure or indirect gravure coating. Such coating equipment is used for pattern coating of silicone to produce complex double release constructions and other specialty products. Vinyl ether monomers such as dodecylvinyl ether and the bis-vinyl ether of 1,4-cyclohexanedimethanol have been shown to be useful reactive diluents for photocurable epoxysilicone release coatings, capable of efficient reduction of viscosity and enhancement of photocure, as taught by Eckberg et al in U.S. Pat. No. 5,650,453. However, the presence of vinyl ethers in the cured epoxysilicone release coating gives rise to unstable release of the silicone coated liner from most acrylic and rubber based adhesives. Alkylphenols such as 4-dodecylphenol (DDP), in blends with certain alkanediols, have been shown to be effective reactive diluents for cationic photocure epoxysilicone release systems, as taught by Descorcie et al in U.S. Pat. No. 5,010,118. Also, the use of DDP or other alkylphenols in such cases for the enhanced miscibility of alkoxy-substituted bis-aryl iodonium and tris-phenyl sulfonium hexafluoroantimonates has been published. Phenol-substituted epoxysilicone terpolymers have been described as controlled release additives for UV epoxysilicone systems in Riding et al, U.S. Pat. No. 4,952,657.
The present invention is based on the discovery that certain specific combinations of additives are miscible with photocurable silicone compositions and also act as compatibilizing agents for polar ""onium type cationic photocatalysts which are often incompletely miscible with nonpolar organofunctional silicones and which, unexpectedly, provide dramatic improvement in the stability of release of the resultant UV cured silicone coating from aggressive acrylic adhesives. The present invention provides for photocurable silicone release coating compositions that are efficiently photo-crosslinked, readily applied to film, paper, and glassine substrates, and which provide exceptionally stable release from aggressive adhesives. The compositions of the instant invention provide for a curable additive for catatonically photocurable silicone polymers, said additive selected from a first group consisting of vinyl ether monomers and a second group selected from a group of alkylphenol compounds.
The present invention therefore provides for an ultraviolet light or electron beam curable silicone coating composition comprising:
a) An epoxy functional silicone selected from the group consisting of:
MDxDEyQzTuDRfjM
MEDxDEyQzTuDRfjME
MEDxDEyQzTuDRfjM
And mixtures thereof; and where:
M=(CH3)3SiO1/2 
ME=(C6H9O(CH2)2)(CH3)2SiO1/2 
D=(CH3)2SiO2/2 
DE=(C6H9O)(CH2)2(CH3)SiO2/2 
DRf=(CF3CH2CH2)(CH3)SiO2/2 
Q=SiO4/2 
T=(CH3)SiO3/2 
where j, x,y,z, and u are positive integers and j, z, and u may be zero and wherein said epoxy-functional silicone has a viscosity ranging from about 20 to about 100000 centistokes at 25xc2x0 C. And, if preferred, a polydimethylsiloxane chainstopped with (poly)(carbinol) group-containing trialkylsiloxy species of the general formula:
MBOHDvMBOH
Where D is as defined above, and MBOH=R1R2R3SiO1/2, with R1, R2, and R3 each monovalent radicals and each independently selected from the group consisting of hydrogen, trifluoropropyl, monovalent hydrocarbon radicals, and (HO-alkyl)nxe2x80x94 radicals such that each MBOH chainstopper unit includes at least one, and preferable more than one, (HO-alkyl) group;
b) an unsaturated ether compound selected from the group consisting of:
CH2xe2x95x90CHxe2x80x94Oxe2x80x94(CH2)11CH3 
(CH2xe2x95x90CHxe2x80x94Oxe2x80x94CH2)2xe2x80x94(C6H10)
CH2xe2x95x90CHxe2x80x94Oxe2x80x94(CHa-bRxe2x80x2bCH2xe2x80x94Oxe2x80x94)cxe2x80x94CHxe2x95x90CH2 
(CH2xe2x95x90CHxe2x80x94Oxe2x80x94CH2)(HOCH2)(C6H10)
CH2(CH2COOCH2xe2x80x94C6H10xe2x80x94CH2OCHxe2x95x90CH2)2 
(CH2xe2x95x90CHxe2x80x94Oxe2x80x94(CH2)4OOC)2(C6H4)
CF3(CF2)dxe2x80x94Oxe2x80x94CHxe2x95x90CH2 
Where a is two, b is zero or 1, c and d are integers, and Rxe2x80x2 is an alkyl group selected from CH3, C2H5, C3H7, and C4H9;
c) an alkylphenol selected from the group described by:
Rwxe2x80x94C6Hi(OH)k, where R=saturated or unsaturated aliphatic substituent of from 1 to about 20 carbon atoms, halogenated aliphatic groups, alkylaryl groups, and where i=4xe2x88x92(w+k);
d) an effective amount of a bis(alkylphenyl)iodonium salt photocatalyst, said photocatalyst being selected from the salts of the groups of acids consisting of hexafluoroantimononic acid, hexafluorophosphosphoric acid, hexafluoroarsenic acid, tetrafluoroboric acid, triflic acid, tetra(perfluorophenyl)boric acid and mixtures thereof.
The subscripts on the various components of the epoxy-functional silicones may be varied within the constraints listed such that viscosity and other properties are within a desired range. The present invention further provides for curable silicone compositions that include unsaturated ethers selected from the group listed in b) above, plus alkylphenols selected from the group listed in c) above and further defined as 4-C12H25xe2x80x94(C6H4)xe2x80x94OH and 4-C9H19xe2x80x94(C6H4)xe2x80x94OH, and 2-C3H5xe2x80x94(C6H4)xe2x80x94OH. The present invention comprises ingredients a), b), c), and d). Those knowledgeable and skilled in the art will recognize that other epoxy-functional silicones bearing various other organofunctional groups such as polyethers, hydroxyesters, and the like, may be substituted for the epoxysilicone ingredients a) listed above.
UV curable siloxane fluids provided by the instant invention are more specifically dialkylepoxysiloxy- or trialkylsiloxy chain-stopped polydialkyl-alkylepoxysiloxane copolymers diluted with a reactive vinyl ether monomer plus an alkylphenol. The reactive vinyl ether monomer is independently polymerizable and crosslinkable by virtue of the presence of vinyl ether groups in the monomer, while the alkylphenol will cure into the crosslinked epoxysilicone network. Thus, curing of this combination system as provided by the present invention results in interpenetrating polymerized networks that may or may not be crosslinked to each other.
The epoxy functionality is obtained when certain of the hydrogen atoms on the polysiloxane chain of a polydialkyl-alkylhydrogen siloxane copolymer are reacted with unsaturated epoxide molecules via hydrosilation addition as taught in U.S. Pat. No. 5,258,480.
The radiant energy curable epoxyfunctional silicone fluid can comprise an epoxy-functional dialkyl-alkylsiloxy- or trialkylsiloxy-chainstopped polydialkyl alkylepoxy siloxane copolymer fluid that is the product of sequential hydrosilation reactions involving a polydialkylsiloxane that includes unsaturated groups with a separate polydialkyl-alkylhydrogen siloxane, followed by reaction of an unsaturated epoxy monomer wherein the resulting epoxy-silicone fluid product is subsequently diluted with a vinyl ether plus alkylphenol.
The alkyl groups of the pre-crosslinked polydialkyl-alkylepoxy siloxane are preferably methyl groups. The unsaturated polydialkylsiloxane is preferably a vinyl-dimethylsiloxy chain stopped linear PDMS. The unsaturated epoxy monomer used to make the epoxy functional silicone is preferably an olefinic cyclo-aliphatic epoxy compound such as 4-vinylcyclohexenemonoxide (VCHO), vinylnorbornenemonoxide, limoneneoxide, or dicyclopentadiene monoxide.
Hydrosilation reactions are used for pre-crosslinking and subsequent functionalization of silylhydride containing PDMS. These reactions are catalyzed by low levels of Group VIII noble metal complex compounds. Such complexes are silicone-miscible compounds of ruthenium, rhodium, palladium, platinum, osmium, and iridium. Certain rhodium and platinum complex compounds are particularly preferred, as taught by Eckberg et al in U.S. Pat. No. 5,391,676.
The vinyl functional silicone employed in the pre-crosslinking network synthesis may be selected from the group consisting of dimethylvinylsiloxy-chainstopped polydimethylsiloxane, dimethylvinyl-chainstopped dimethyl-methylvinyl siloxane, tetravinyl tetramethylcyclotetrasiloxane, and sym-tetramethyldivinyldisiloxane.
The hydrogen functional siloxane precursor fluid can be selected from the group consisting of dimethylhydrogensiloxy-stopped polydimethylsiloxane, dimethylhydrogen siloxy-stopped polydimethyl-methylhydrogen siloxane, trimethylsiloxy-stopped polydimethyl-methylhydrogen siloxane, tetramethylcyclotetrasiloxane, and sym-tetramethyldisiloxane.
Certain vinyl ether monomers and oligomers are widely available, and, as cationically polymerizable monomers and oligomers, have been found to be useful additives and diluents in compatible photocurable epoxy-based compositions. Such compositions have been found to be curable by means of electron beam radiation as well as by ultraviolet light. The reactive additives consist in part of vinyl ether monomers and oligomers, with those most preferred for the practice of the instant invention being:
CH2xe2x95x90CHOxe2x80x94(CH2)11CH3,
(CH2xe2x95x90CHOxe2x80x94CH2)2xe2x80x94(C6H10),
(CH2xe2x95x90CHOCH2)(HOCH2)(C6H10),
CH2(CH2COOCH2xe2x80x94C6H10xe2x80x94CH2OCHxe2x95x90CH2)2,
(CH2xe2x95x90CHO(CH2)4OOC)2(C6H4),
CH2xe2x95x90CHO(CHaxe2x88x92bRxe2x80x2bCH2O)cxe2x80x94CHxe2x95x90CH2,
CF3(CF2)dxe2x80x94OCHxe2x95x90CH2,
where a is 2, b is 0 or 1, c and d are integers, and Rxe2x80x2 is an alkyl group selected from CH3, C2H5, C3H7, and C4H9,
and also consist in part of alkylphenol compounds including but not limited to:
4-(C12H25xe2x80x94(C6H4)xe2x80x94OH,
4-(C9H19xe2x80x94(C6H4)xe2x80x94OH,
2-(CH2xe2x95x90CHCH2)xe2x80x94(C6H4)xe2x80x94OH.
The chemical formulas of the compounds in the foregoing lists are written without regard for positional isomerism of substituents. Since the polymerizing and crosslinking of these molecules depends only on the reactive vinyl ether and phenol groups of the vinyl ether monomers and oligomers and alkylphenols, respectively, the relative positional isomerism of these reactive substituents should not affect the chemical properties for which these compounds are used in the instant invention. Therefore these compounds are claimed as the general structures with the expectation that geometric isomers having the same empirical and molecular formula will function similarly to effect the purposes of the invention.
We have discovered that when the vinyl ether+alkylphenol diluted epoxy functional silicone polymers (also referred to as epoxysilicones) are combined with a compatible bis-aryl iodonium catalyst, the resulting mixture can be exposed to ultraviolet light or electron beam radiation to initiate a crosslinking reaction which in turn forms a solid adhesive silicone release coating incorporating the photo-polymerized vinyl ether and alkylphenol possessing a very stable release from common aggressive pressure sensitive adhesives (PSA""s) to which the photocured silicone coating is laminated.
UV light curable or electron beam (EB) curable epoxysilicone compositions of the instant invention may be applied to cellulosic or plastic film substrates including but not limited to supercalenderedkraft (SCK) paper, glassine paper, polyethylene kraft (PK) paper, polyethylene film, polypropylene film, polyester film, and polyvinylchloride (PVC) films. The actinic radiationxe2x80x94initiated crosslinking reaction cures the liquid coating to form a solid, crosslinked non-adherent (or adhesive) surface also known as a release coating on the substrate so coated.
It has been shown that UV and/or EB curable epoxysilicone polymers such as those taught by Eckberg et al in U.S. Pat. No. 4,279,717 and in subsequent patents, are efficiently cured in the presence of certain compatible ""onium type cationic photocatalysts without inhibition by atmospheric oxygen, which is a useful feature of this technology, since practice of such coating processes does not require complex xe2x80x9cinertingxe2x80x9d of cure chambers. Such epoxysilicone compositions are constrained within a narrow range of viscosity and epoxy content dictated by the need for application of defect-free coatings about 0.5 to 2.0 microns thick capable of application on substrates of choice at high speed, and by the necessity that these photocurable epoxysilicone coatings quickly photo-crosslink upon exposure to actinic radiation, all the while retaining good adhesion to the substrate.
Three roll offset gravure or multi-roll film splitting coating techniques are commonly practiced for high speed coating of silicone compositions and are most suited for application of silicones whose viscosity is in the range of 100 to 3000 cstk viscosity at the temperature of application. Rapid cure requires sufficient amount of reactive oxirane be present in the epoxysilicone to ensure high reactivity of the coating, as well as aid in dissolution of the polar ""onium salt catalyst in the coating. But, if too much oxirane is present in the epoxysilicone, the cured coating will not properly release PSA""s.
The epoxy-functional silicones comprising the coating composition of the instant invention are selected from the group consisting of:
MDxDEyQzTuDRfjM,
MEDxDEyQzTuDRfjME,
MEDxDEyQzTuDRfjM,
and mixtures thereof, where:
M=(CH3)3SiO1/2,
ME=(C6H9O)(CH2CH2)(CH3)2SiO1/2 
D=(CH3)SiO2/2 
DE=(C6H9O)(CH2CH2)(CH3)SiO2/2 
DRf=(CF3CH2CH2)(CH3)SiO2/2 
Q=SiO4/2 
T=CH3SiO3/2 
where j, u, x, y, and z are positive integers and j, z, and u may be zero and where said epoxy-functional silicone has a viscosity of between 20 and 100000 cstk at ambient conditions. The subscripts on the various components of the epoxysilicones may be varied at will within the constraints already listed such that the polymer viscosity is within the defined, coatable range. It should be noted that the viscosity of these undiluted epoxysilicones can exceed that which may be readily applied to substrates using conventional application equipment. If a diluent is used to improve the coating characteristics of the epoxysilicone composition it is important that the selected diluent or blend of diluents be capable of reducing the viscosity of the coating composition while maintaining the crosslink density and photoresponse of the curable coating mixture. An additional consideration regarding choice of diluent is that the curing catalyst (an ""onium salt for such cationically polymerizable compounds) must remain miscible with the coating bath. It is known that certain vinyl ethers and vinyl ether oligomers both reduce viscosity and accelerate the photocure of epoxy-based coating compositions. In addition, the inclusion of a second reactive diluent may further advantageously speed crosslinking and improve catalyst compatibility. Thus, the specific combination of vinyl ether and alkylphenol diluents of the instant invention provide for a novel silicone-based coating system that permits use of high viscosity epoxysilicone or of epoxysilicones that would otherwise be immiscible with polar ""onium photocatalysts but for the addition of the vinyl ether and the alkylphenol. Since the coating composition is a solvent-free composition as defined in the art, the fraction of vinyl ether monomers and oligomers and alkylphenols present in the coating mixture can range from about 1 to 90 percent, more preferably from about 2 to 50 percent, and most preferably from about 5 to 25 percent. Based on a coating composition of 100 parts containing about 5 to 20 parts total reactive diluent mixture and 95 to 80 parts epoxy-functional silicone, 0.2 to 3 parts of cationic-type ""onium salt photocure catalyst are added, more preferably 0.2 to 2 parts and most preferably 0.2 to 1.5 part.
The requirement for rapid efficient photocure militates that photocatalysts and, if desired, photosensitizers, be freely miscible with the photocurable compositions in which they are mixed, forming either clear solutions or stable suspensions or dispersions. In the case of epoxy-functional photocurable silicones of the instant invention, ""onium-type cationic salt photocatalysts should be compatible with the epoxysilicone fluid. Iodonium salts of the general formula (R-Ph)2I+Xxe2x88x92 have been designed to address the miscibility issue posed by use of ionic polar substances in nonpolar silicone polymers, where R is typically a mixture of alkyl fragments derived from linear alkylate grade dodecylbenzene; such R is generically termed xe2x80x98dodecylxe2x80x99 although the alkyl groups vary in chain length. As a consequence, the (R-Ph)2I+ salts are mixtures rather than pure compounds and therefore exhibit freezing point depression relative to a pure bis-alkylphenyl iodonium salt and are amorphous, non-crystalline semi-fluids that are more compatible with nonpolar media especially epoxysilicones. Where X=SbF6xe2x88x92 or B(C6F5)4xe2x88x92, the iodonium species are most compatible with epoxysilicone polymers and are most efficient at promoting rapid photocrosslinking reaction; where X=PF6xe2x88x92, Clxe2x88x92 or BF4xe2x88x92 or other similarly small anions whose charge is localized, analogous iodonium salts are poorly compatible with epoxysilicone polymers useful for silicone release coatings. It should be noted that the iodonium salts where the anion is hexafluoroantimonate or tetrakis(perfluorophenyl)borate, while very effective for photocure of epoxysilicone release coatings, are much more expensive than iodonium salts whose anion is hexafluorophosphate or tetrafluoroborate. It is therefore evident that if inexpensive iodonium salts can be made useful for photocure of epoxysilicone coatings in which they are normally immiscible, it would be advantageous for processing of photocurable release coatings.
The UV curable epoxy-functional silicone based compositions of the instant invention can be applied to cellulosic and other substrates including paper, metal, foil, plastic, films, PK paper, SCK paper, and other common liner materials. A UV light or electron beam initiated reaction will cure the epoxysilicone compositions of the present invention to form a non-adherent, adhesive surface on the coated substrate.
Ultraviolet light curable silicone release coating compositions of the instant invention are obtained by combining an iodonium salt effective for catalysis of a UV light initiated reaction of a cationically polymerizable material with a dialkylepoxysiloxy- or trialkylsiloxy-stopped epoxysilicone fluid having a viscosity of about 20 to 100000 cstk at 25xc2x0 C. with a vinyl ether monomer plus an alkylphenol compound.
The preferred UV light initiator or photocatalyst utilized by the present invention is a diaryl iodonium salt derived from xe2x80x98linear alkylatexe2x80x99 dodecylbenzene. Such salts have the following general formula:
[(CxH2x+1)Ph]2I+Yxe2x88x92
where x is an integer varying from about 6 to about 18, Y=SbF6, AsF6, PF6, BF4, or B(C6F5)4xe2x88x92, and Ph stands for a phenyl group. xe2x80x98Linear alkylatexe2x80x99 dodecylbenzene is a commercial grade of alkylbenzenes prepared by Friedel Craft alkylation of benzene with a mixed alpha-olefin distillation cut. Consequently, the alkylate contains both straight and branched chain alkyl benzenes of a wide range of molecular weight and isomer mix. Such a mixture is useful, however, to promote compatibility of the iodonium salt derived therefrom with nonpolar epoxysilicones.
The UV curable silicone coating compositions of the present invention utilize epoxy-functional silicone fluids that can be prepared via different routes. Olefin epoxide compounds that include unsaturation and oxirane functionality react with silyl-hydride functional polysiloxanes via hydrosilation to produce organofunctional siloxanes bearing pendant epoxy groups. 4-vinylcyclohexeneoxide (VCHO) is especially useful for synthesis of UV curable epoxysilicones in this fashion. The analogous hydrosilation reaction between vinyl siloxane groups and silylhydride groups is a well-known means of crosslinking silicone coatings and elastomers. This latter reaction is used to partially crosslink silylhydride functional silicones and vinyl-functional silicones creating a lightly crosslinked loose network, which, provided an excess of silylhydride is present at the onset of the reaction, creates a siloxane network with residual SiH groups that can undergo subsequent addition to unsaturated epoxy monomers such as VCHO to produce epoxysilicones which are partially networked prior to final photocure.
Such epoxysilicones can be prepared from other unsaturated oxiranes including allylglycidyl ether, vinylnorbornene oxide, and dicyclopentadiene monoxide. While cyclohexyl epoxy functional silicones are particularly useful, other olefin epoxies may be used to create epoxysilicones without altering the properties of the cured silicone release coating. The scope of this invention is not limited to the epoxy silicone species used in the examples to follow.
The photocurable epoxysilicone based coating compositions comprising the instant invention are illustrated by numerous example which follow. The instant invention is not limited by the particular examples described. Those skilled in the art will be able to provide other epoxy functional silicones, vinyl ether monomers or oligomers, and other alkylphenol diluents upon consideration of the examples. All US patents referenced in this application are hereby and herewith incorporated by reference.